2 Basic JMS API Concepts
This chapter introduces the most basic
JMS API concepts, the ones you must know to get started writing simple
JMS client applications:
The next chapter introduces the JMS
API programming model. Later chapters cover more advanced concepts,
including the ones you need to write J2EETM applications that use message-driven beans.
2.1 JMS API Architecture
A JMS application is composed of the following parts.
- A JMS provider is a messaging system
that implements the JMS interfaces and provides administrative and control
features. An implementation of the J2EE platform at release 1.3 includes
a JMS provider.
- JMS clients are the programs or
components, written in the JavaTM programming
language, that produce and consume messages.
- Messages are the objects that communicate
information between JMS clients.
- Administered objects are preconfigured
JMS objects created by an administrator for the use of clients. The two
kinds of administered objects are destinations and connection factories,
which are described in Section 3.1, "Administered
Objects."
- Native clients are programs that use a
messaging product‘s native client API instead of the JMS API. An application
first created before the JMS API became available and subsequently modified
is likely to include both JMS and native clients.
Figure 2.1
illustrates the way these parts interact. Administrative tools allow
you to bind destinations and connection factories into a Java Naming
and Directory InterfaceTM (JNDI) API namespace.
A JMS client can then look up the administered objects in the namespace
and then establish a logical connection to the same objects through the
JMS provider.
Figure 2.1 JMS API Architecture
2.2 Messaging Domains
Before the JMS API existed, most messaging
products supported either the point-to-point or the publish/subscribe
approach to messaging. The JMS Specification provides a separate domain for
each approach and defines compliance for each domain. A standalone JMS provider
may implement one or both domains. A J2EE provider must implement both domains.
In fact, most current
implementations of the JMS API provide support for both the
point-to-point and the publish/subscribe domains, and some JMS clients
combine the use of both domains in a single application. In this way,
the JMS API has extended the power and flexibility of messaging
products.
2.2.1 Point-to-Point Messaging Domain
A point-to-point (PTP) product or
application is built around the concept of message queues, senders, and
receivers. Each message is addressed to a specific queue, and receiving
clients extract messages from the queue(s) established to hold their
messages. Queues retain all messages sent to them until the messages
are consumed or until the messages expire.
PTP messaging has the following characteristics and is illustrated in Figure 2.2.
Figure 2.2 Point-to-Point Messaging
- Each message has only one consumer.
- A
sender and a receiver of a message have no timing dependencies. The
receiver can fetch the message whether or not it was running when the
client sent the message.
- The receiver acknowledges the successful processing of a message.
Use PTP messaging when every message you send must be processed successfully by one consumer.
2.2.2 Publish/Subscribe Messaging Domain
In a publish/subscribe (pub/sub)
product or application, clients address messages to a topic. Publishers
and subscribers are generally anonymous and may dynamically publish or
subscribe to the content hierarchy. The system takes care of
distributing the messages arriving from a topic‘s multiple publishers
to its multiple subscribers. Topics retain messages only as long as it
takes to distribute them to current subscribers.
Pub/sub messaging has the following characteristics.
- Each message may have multiple consumers.
- Publishers
and subscribers have a timing dependency. A client that subscribes to a
topic can consume only messages published after the client has created
a subscription, and the subscriber must continue to be active in order
for it to consume messages.
The JMS API relaxes this timing dependency to
some extent by allowing clients to create durable subscriptions. Durable
subscriptions can receive messages sent while the subscribers are not active.
Durable subscriptions provide the flexibility and reliability of queues but
still allow clients to send messages to many recipients. For more information
about durable subscriptions, see Section 5.2.1,
"Creating Durable Subscriptions."
Use pub/sub messaging when each message can be processed by zero, one, or many consumers. Figure 2.3 illustrates pub/sub messaging.
Figure 2.3 Publish/Subscribe Messaging
2.3 Message Consumption
Messaging products are inherently
asynchronous in that no fundamental timing dependency exists between
the production and the consumption of a message. However, the JMS
Specification uses this term in a more precise sense. Messages can be
consumed in either of two ways:
- Synchronously. A subscriber or a
receiver explicitly fetches the message from the destination by calling
the
receive
method. The receive
method can block
until a message arrives or can time out if a message does not arrive within
a specified time limit.
- Asynchronously. A client can register a
message listener with a consumer. A message listener is similar to
an event listener. Whenever a message arrives at the destination, the JMS
provider delivers the message by calling the listener‘s
onMessage
method, which acts on the contents of the message.